The present invention relates to a locking device for telescopic sections of a telescopic jib, and more particularly to the jib of a crane or mobile crane having such a locking device.
Crane jib telescopic sections are lockable in the extended condition to, among other things, relieve the load on the telescoping system. This applies especially when use is made of entraining means, for example piston/cylinder units, for extending the telescopic sections in bringing one telescopic section after the other into the extended or retracted positions.
For locking telescopic sections use is made mostly of locking pins, which engage from one telescopic section into a receiving portion of the adjoining telescopic section. In earlier practice such pins were retracted from the outer side of a surrounding telescopic section inward into a receiving portion of the telescopic section located within. On the one hand, this is an unsightly disadvantage, since exposed locking units spoil the neat design of the jib as a whole. On the other hand, such conventional jibs and pinning systems necessitate disposing a power supply (hydraulics, pneumatic system, etc) to the pinning means on the outer side of the jib, which is a technical complication to be avoided.
In a very early practice, such as disclosed in U.S. Pat. No. 4,036,372 a locking system with locking pins engage from an inner telescopic section outwards into receiving portions of an outer telescopic section, as a result of which the locking system could be disposed in the interior of the jib, thus obviating the need of locating power supplies on the outer side of the jib. However, the drawback in this system is that each of the locking pins engages the upper or lower plates of the telescopic section, i.e. at locations at which maximum bending stresses (tension at the top, compression below) occurs due to the external loads. In addition to this the release device described in this patent operates with double-acting cylinders and is thus, for this reason alone, relatively complicated.
Known from EP O 661 213 Al is a locking device for telescopic sections in which a locking pin guided in an inner telescopic section is able to engage a receiving portion in an outer telescopic section. This document describes the use of spring-biased locking pins in which the pin is maintained in a locked position by the force of a spring, whilst the inner end of the pin can be engaged by a hydraulic release device to return the pin into the released position in overcoming the force of a spring. A parallel description of the same design is described in U.S. Pat. No. 5,628,416.
In the last-mentioned locking device only a single locking pin is provided for locking two telescopic sections, this locking pin engaging the top flange shells of the jib sections in the middle. Thus, here too, there is again the disadvantage that the passages for the locking pins are provided just in the cross-sectional zone of the jib profile where maximum tensile stresses occur and, therefore, the structure of the jib sections is interrupted at an unsuitable location due to the recesses.